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astronomy

More Interstellar Visitors?

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Yesterday I talked about the latest interstellar intruder to come through our solar system. 2I/Borisov is thought to be a comet from another star system and this interstellar visitor is traveling on a fast trajectory past the Sun. By the end of the year, it will be well beyond Jupiter. So, astronomers are studying it now, while it’s still close enough to observe. The appearance of 2I/Borisov, along with the visit in 2017 from ‘Oumuamua (the first interstellar asteroid detected passing through our solar system), raises a lot of questions about how these objects get cast out on their long journeys.

Planet Formation and Interstellar Visitors

Planetary scientists spend a lot of time working to understand the formation of our own solar system. It turns out that planetary formation likely plays a role in the origins of alien interstellar objects. The general story is that the planets were built up from smaller objects called planetesimals. Those miniature worlds accreted from rocky materials and ice that existed in the infant solar system. This cloud of gas and dust, called the protoplanetary nebula, was the birthplace of the planets, moons, asteroids, and comets. All that material circled the Sun, and, over time, the planets formed and migrated to their present positions.

Of course, comets and asteroids migrated, too. They ended up in two regions in the solar system: the Kuiper Belt and the Oort Cloud. We already know about dwarf planets in the Kuiper Belt, and of course, there are populations of comets in both regions.

In the early solar system, the larger planetary objects swept out “clear lanes” in the dust cloud as they accreted. It turns out the planets that create those lanes may play a role in ejecting objects to space.

This artist’s conception of a protoplanetary disk shows what our own solar system might have looked like some 4.5 billion years ago. Giant planets forming in the disk would have swept out “lanes” in the cloud of gas and dust, and cooler materials (like cometary ices and volatile gases) would migrate away from the heat of the Sun. Gravitational interactions between planets and smaller “rocky debris” in the cloud could have ejected the objects out to interstellar space. Courtesy European Southern Observatory.

Making the Kuiper Belt and Oort Cloud

Planetary scientists want to understand how those comets and other bodies got “stuck” in such far-flung orbits. The details are still being worked out, but here’s the basic story. Interactions in the young solar system between larger worlds caused cometary particles to be “flung” out to the outer regions. Those regions are cooler places, where the icy bodies could more easily exist.

Could they have been “flung out” even farther? Maybe even out of the solar system altogether? Good questions. If they were, then there could be bits of our own solar system traversing the depths of space, just as 2I/Borisov and ‘Oumuamua are doing.

How Do Interstellar Visitors Get “Launched”?

A pair of astronomers at Yale University, Gregory Laughlin and Malena Rice, have been studying the situation. They are interested in the dynamics of just how interstellar objects could be ejected from their newly forming planetary systems. Their idea is simple. Travelers from a newborn solar system could be material ejected from large, newly formed planets in orbit around a star. Those planets, which would likely be orbiting farther away from their star, would have carved out pronounced gaps in the protoplanetary disks.

Far from being a quiet, peaceful place, those disks are very active. They’re made of very dense clouds of gas and dust, and all that stuff mixes and collides. The gravity of the star also disturbs the disk, as does the activity of magnetic fields. Some of that action helps build new planets. However, some of it could also cause gravitational interactions between large, distant planets and the material around them. Those actions could fling material out of the newly forming solar system at a very high speed.

If that happens, then the ejected bodies would travel through space. Eventually, they could pass through neighboring planetary systems, just like 21/Borisov and ‘Oumuamua have done.

So, can such actions be observed? It’s possible, as astronomers are finding out.

Intruder Homes are Out There

The two scientists wanted to see if their idea works out. First, they got time on the Atacama Large-Millimeter Array in Chile to look around at some nearby young planetary systems. ALMA is a linked collection of radio telescopes tuned to submillimeter wavelengths of light. That kind of light can slip through the dusty disks surrounding other stars and reveal information about their characteristics.

An image of a protoplanetary disk, from the Atacama Large Millimeter/submillimeter Array telescope in Chile. The black interior rings are gaps in the disk. Credit: ALMA (ESO/NAOJ/NRAO), S. Andrews, et al.; N. Lira

The astronomers looked at three protoplanetary disks, searching out large gaps made by distant planets as they plow through their birth disks.

Next, the pair applied what they observe to computer simulations of disks with gaps. They are coming up with some boundaries on the sizes of such objects being flung away from the systems they observe. It turns out, based on observations and simulations that giant planets with lengthy periods (the time they take to orbit their stars) and that create gaps, can gravitationally eject objects the size of ‘Oumuamua. That asteroid is somewhere between 100 and 1000 meters long and 35 to 170 meters across. That may be out our 2017 visitor got started on its long trip through the galaxy!

Implications of Interstellar Visitors

As I mentioned yesterday, close studies of these intruders from other star systems give clues to the conditions in their home systems. Thanks to the work by Laughlin and Rice, we now know that the protoplanetary disks themselves have information about the sizes, shapes, and numbers of those intruders they send out. Based on the work by this pair of scientists, it turns out we can expect MORE intruders in the coming years. With a new generation of telescopes able to study these interstellar travelers in more detail, it looks like we’ll be getting a whole new look at distant star systems and their planetary processes.

You can read more about Laughlin and Rice’s work in their paper.

astronomy

2I/Borisov: Another Interstellar Visitor Flashes By

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Space isn’t really so empty as we might think. There are planets, moons, asteroids, comets, and rings. And dust. Beyond our local neighborhood, there’s also gas, lots of it. And, of course, we get the errant comet or asteroid from another solar system, like the newly found 2I/Borisov.

Earlier this year, an amateur astronomer named Gennadiy Borisov was out observing. On August 30, he found something that looked like a comet. He reported his find in a notification to the IAU. Other observers went to work looking at it, too. Using their observations, the IAU Minor Planet Center came up with a preliminary orbit. It turns out this thing isn’t just any comet. It’s coming from outside the solar system. Its official designation means it’s the second interstellar object and it was discovered by Borisov.

2I/Borisov is Hyperbolic!

This alien comet is in a hyperbolic orbit. That means its origin is from outside the solar system and it will (unless captured gravitationally by the Sun (unlikely), end up sailing right through our planetary system and back out to interstellar space. A hyperbolic orbit is not a “closed” orbit. That is, it isn’t an ellipse (like a squashed egg) It’s open-ended. (Geometry students will remember hyperbolas from their studies of ellipses, arcs, and so on).

A schematic of 2I/Borisov’s very hyperbolic orbit. Courtesy NASA/ESA/STScI.

Objects that follow such orbits into the solar system seem rare. They may not be as numerous as stuff from within our region of space. It’s also possible that we’ve spotted them before, but didn’t realize their origins. But, it makes sense that they’re out there. Planetary systems lose objects in the normal course of their evolution. So, there’s no reason some of them aren’t coming this way for a quick fly-by.

Recall that in 2017, astronomers spotted ‘Oumuamua. This is an asteroid-type object also on a hyperbolic orbit. It passed within 38,624,256 kilometers (24 million miles) of the Sun before heading back out to interstellar space. Planetary scientists are still studying data taken during its flyby.

Hubble Space Telescope Looks at 2I/Borisov

The Hubble Space Telescope’s first look at 2I/Borisov, taken October 12, 2019. Courtesy NASA/ESA/STScI/D.Jewitt.

The Hubble Space Telescope studied 2I/Borisov as it swept past Earth at a distance of 418,429,440 kilometers (260 million miles). It will continue to observe for a few months as the comet makes its way out past Jupiter and to interstellar space.

Unlike ‘Oumuamua, which really didn’t exhibit much in the way of cometary characteristics, 2I/Borisov looked more like a comet. It had a central nucleus, surrounded by a dust cloud (coma). A spectroscopic study (made by dissecting the light from the comet) should give astronomers info about its chemical composition. They should also be able to get a good idea of the type of star system where it was born.

With a better idea of its path through space, eventually, they might be able to figure out WHERE it came from. However, that’s not what HST was programmed to do. Its instruments are focused on imaging and data about the object itself. The best way to figure out the origin of an interstellar visitor is to trace its trajectory as far as possible. With that data, astronomers can extrapolate a very probable path through space. If there’s a star system along the way, that might be its home. However, we may never know for sure. Things are in constant motion in our galaxy. An object from a distant area could well have its trajectory changed over long periods of time.

Need More Data about 2I/Borisov!

Hubble observations, as well as studies using other telescopes, will be used to help astronomers in an other way. Information about this comet could help them understand the origins of comets and other small bodies. We know where they are in our current solar system (the Oort Cloud and Kuiper Belt). Are they in the same places in other star systems? How do they evolve over time? What can our Kuiper Belt tell us about other system’s “cometary belts“?

Astronomers and planetary scientists are constructing very accurate models of how these small bodies formed in the early solar system. They also look at how comets and asteroids scattered around. Do they do the same thing at other stars? This comet’s data will help answer that and other questions.

A Peek at a Neighbor’s Business

Astronomers know the general properties of stars. They can deduce some information about their planetary systems just by studying orbital characteristics of the star and its planets. However, the key to understanding their chemistries is in spectroscopic data. Spectroscopy of light from objects is like getting a peek into the characteristics of the systems where they exist. In this case, spectroscopy of 2I/Borisov will lift the veil on conditions in its home system.

So, think of this comet as a treasury of clues about where it originated. While the comet is still in OUR solar system, astronomers will continue to study it with all the modern tools. Eventually, however, the comet will leave the solar system, leaving behind a great deal of dust, information, and clues to its origins and makeup. Eventually, what we learn from it will help us understand the conditions at our stellar neighbors.